This invention relates in general to a method of coating an optical fiber, and in particular to such a method that is carried out immediately after the fiber is drawn and that results in maintaining the mechanical strength of the optical fiber after the fiber has been drawn and especially during its storage and use.
Typical tensile strengths observed for the material of silicon oxide optical fibers at the time the fibers are drawn under ideal conditions are on the order of 1 million p.s.i. In certain optical communications, it is necessary to employ optical fibers having lengths above 1 kilometer. The problem in the art when using such long lengths of optical fibers has been the lack of adequate mechanical strength of the fiber as such. That is, the tensile strength of long lengths of commercially available optical fibers is in the range of 50,000 to 80,000 p.s.i. A mechanical strength above 200,000 p.s.i., however, is needed for optical fibers when used as optical waveguides in certain specialized applications, as for example, in rapid payout communication systems employing long lengths of fiber.
One reason why long lengths of fibers have not previously been prepared with sufficient mechanical strength was the presence of submicron surface flaws caused either by light mechanical abrasion during and after the usual fiber drawing operation and/or by chemical attack by atmospheric contaminants such as moisture. Attempts to solve these problems have been made by applying organic coatings to these fibers after the fibers have been drawn. However, these organic coatings have not been impervious to moisture or hydroxyl ion diffusion. This has led to the reduced strength of organic coated fibers during periods of use or storage. Optical glass fiber is very susceptible to moisture and many hostile environments. Therefore, the fiber needs a hermetic coating to protect its structural integrity.
One of the most feasible methods currently used for coating glass fibers with inorganic materials, such as silicon or various metals, is by chemical vapor deposition (CVD).
In chemical vapor deposition, the material of the coating is formed in a gaseous state, either by releasing the material of the coating from a single gaseous reactant at a temperature needed for releasing such a material, or by reacting at least two gaseous reactants with one another at the required reaction temperature. The material of the coating forms particles in the gaseous medium in which the release or the chemical formation of such material takes place. These particles are then deposited on the exposed surface of the fiber.
This approach has an inherent disadvantage in that the CVD reaction products (mostly solid particles) can interact with the substrate surface, that is, with the exposed surface of the fiber, to produce surface damage during the deposition process. Experience has shown that CVD usually produces large grains or growth cones which can produce substantial flaws in the fiber to cause degradation in strength of the fiber, by creating fissures or microcracks in the material of the fiber. Thus, when this method of coating is used, the strength of unit length of the fiber is reduced below that which the unit length had immediately after the drawing of the fiber or the fiber core and prior to the application of the hermetic coating to the core in the chemical vapor deposition process. While this degradation in strength may not affect each unit length, it occurs often enough for the overall strength of the substantial length of the hermetically coated fiber to be considerably reduced relative to the strength which the material of the fiber had immediately after drawing. This, of course, is very disadvantageous.